This invention relates to spacecraft attitude control, and more particularly, to a sensor on a spacecraft instrument for providing a signal indicating that the instrument is subject to being exposed to undesirable direct sunlight.
Earth and space sensing from spacecraft require protection of sensors from potential damage due to solar radiation impinging on the sensors. For example, for certain sensitive instruments, it is necessary to maintain extremely low temperatures on payload apertures and coolers. Sensors are often mounted on a planar outside surface of a spacecraft without a protective shutter or the like in order to conserve space.
As the low temperature requirements become more stringent, the effects of direct and indirect incident solar radiation on sensors and cooling apparatus become critical.
It has been conventional practice to place the cooler of a satellite on the intended north face such that the spacecraft under normal control, acts as a shield, which prevents sun incidence into the cooler. However, super-cooled payloads require new technology to avoid indirect sunlight during all times during a mission. Direct sun incidence during such common maneuvers as stationkeeping or uncommon maneuvers relating to failures, loss of lock and the like, can damage delicate instruments irreparably.
What is needed is a new technique for avoiding incident solar radiation which protects an instrument sensor or cooler during normal operation and during most types of failures and which does not introduce an undesired weight penalty.
The following patents were uncovered in a search of the records of the United States Patent and Trademark Office in respect to the subject invention.
U.S. Pat. No. 3,348,374, to Schalkowsky, describes a sun-referenced orienting device which uses the sun to provide torques that will orient a spacecraft. As a result, it either points directly at the sun or away from the sun or in any preset angle therebetween. However, it is a type of device which carries an undesired weight penalty, which can be critical in space.
U.S. Pat. No. 3,427,453, to Gill et al., discloses a spacecraft having first sensors which normally provide signals in determining the attitude reference of the spacecraft until interfering solar radiation strikes certain sensors. At such a time, additional sensors are used to determine the attitude reference. The first sensors provide virtually no information regarding location of the source of radiation.
U.S. Pat. No. 3,439,884, to Slater, describes a device using the sun as a heliocentric reference point and a set of stars as another reference point to determine three-axis heliocentric attitude of a spacecraft. As with other references, the '884 patent relies on technology intended to seek out and focus on the sun.
U.S. Pat. No. 4,371,135, to Keigler, describes a system for collecting solar energy by directing the solar rays via one fixed mirror and one rotating mirror in order to direct sun rays to a solar panel at all times. This patent is cited for background showing other technologies.
U.S. Pat. No. 4,375,878, to Harvey et al., describes a satellite having a payload which may be independently oriented with respect to the main body of the satellite. Reference is made to use of either a star tracker or a sun sensor in a conventional method of centering the sensor on the reference.
U.S. Pat. No. 4,508,297, to Mouilhayrat et al., shows a system for arranging solar arrays at canted angles to obtain benefits for certain types of spacecraft.
U.S. Pat. No. 4,674,715, to Frisch applies specifically to a spacecraft platform which carries out one revolution in the orbit direction per orbit. The use of sensors on the platform itself is disclosed so as to improve the pointing performance of the platform and to avoid errors between the main body and the platform. The apparatus requires an accurate pointing device, possibly using the sun.